Intracellular retention of hepatitis B virus surface proteins reduces interleukin-2 augmentation after genetic immunizations

We have previously shown that hepatitis B virus (HBV) surface antigens (HBs Ags) are highly immunogenic after genetic immunization. Compared to the sec reted middle HBV surface proteins (MHBs) or small HBV surface proteins (SHB s), the nonsecreted large HBV surface protein (LHBs), however, induced sign ificantly weaker humoral and cellular immune responses that could not be au gmented by genetic coimmunizations with cytokine expression plasmids, In or der to understand the mechanisms underlying this phenomenon, we examined th e effect of coimmunizations with an interleukin-2 (IL-2) DNA expression pla smid on the immunogenicity at the B- and T-cell level of nonsecreted wild-t ype LHBs, a secreted mutant LHBs, wild-type SHBs, and a nonsecreted mutant SHBs. Coimmunizations of mice with plasmids encoding wild-type SHBs or the secreted mutant LHBs and IL-2 increased anti-HBs responses, helper T-cell p roliferative activity and cytotoxic T-lymphocyte killing. By contrast, coim munizations of plasmids encoding wild-type LHBs or nonsecreted mutant SHBs and IL-2 had no significant effects on immune responses, Interestingly, mic e immunized with cytokine expression plasmids 14 days after the injection o f the wild-type LHRs plasmid showed augmented immune responses compared to animals simultaneously injected with both expression constructs. Anti-HBs r esponses in mice injected with plasmids encoding secreted forms of HBsAgs w ere detectable about 10 days earlier than those in mice immunized with plas mids encoding nonsecreted forms of HBsAgs. Based on these observations, we conclude that cytokines produced by DNA plasmids at the initial site of ant igen presentation cannot augment LHBs specific immune responses because LHB s is not produced at high enough levels or is not accessible for uptake by antigen-presenting cells.